1. Field of the Invention
The present invention relates to a method of producing a honeycomb structure. More particularly, the present invention relates to a method of producing a large honeycomb structure such as a honeycomb structure installed in a large-sized automobile or a honeycomb structure that is divided into a plurality of structures to form final products.
2. Discussion of the Background
A honeycomb structure that is used as a carrier for an automotive exhaust gas purification catalyst is normally produced by extrusion in order to improve mass productivity. Therefore, a forming aid (e.g., binder) and the like are used in addition to the raw material for the base material.
As the binder, cellulose derivatives such as methyl cellulose and hydroxypropyl methyl cellulose have been used. In particular, methyl cellulose that exhibits high gel strength has been suitably used when forming a honeycomb structure using a ram extruder.
However, since a cellulose binder that is currently used normally contains low-molecular-weight components, the low-molecular-weight components contained in the binder may be evaporated together with steam when drying the formed body, cooled and condensed at the inside periphery of the dryer, and adhere to the inside of the dryer. In this case, the adhering low-molecular-weight components may contaminate the dryer, or may be removed from the dryer during drying to adhere and contaminate the product. When the amount of adhering low-molecular-weight components increases to a large extent, the low-molecular-weight components may ignite along with an increase in temperature so that a fire may occur. In order to prevent such a problem, it is necessary to periodically clean the dryer so that the production efficiency and the work efficiency decrease.
If the average molecular weight of the binder is too high, the resulting clay may become hard and extrusion may become difficult. Therefore, a binder which is normally used has been prepared by blending celluloses that differ in molecular weight to achieve the desired average molecular weight. However, since such a cellulose binder contains low-molecular-weight components in order to achieve the desired average molecular weight, the low-molecular-weight components are necessarily evaporated together with steam during drying. This results in the above problem.
When using methyl cellulose as a binder, the fluidity of the clay may decrease if the amount of methyl cellulose to be added is small, so that extrusion may become difficult. Moreover, deformation or cracks may easily occur during drying due to a decrease in dry strength. If the amount of methyl cellulose to be added is large, the amount of heat produced by combustion of methyl cellulose may increase during firing so that cracks may easily occur in the honeycomb structure.
When producing a large honeycomb structure that is installed in a heavy-duty truck or the like, low-molecular-weight components are evaporated together with steam during drying when using methyl cellulose that contains a large amount of low-molecular-weight components. As a result, cracks may easily occur during drying due to a decrease in strength. Moreover, a variation in dimensional accuracy or the like may occur so that the yield of the fired honeycomb structure (final product) may decrease.
Japanese Patent No. 3074169 and Japanese Patent No. 3227039 disclose measures aimed at solving the above problems.
Japanese Patent No. 3074169 aims at providing a method of producing a cordierite ceramic honeycomb structure that is suitable for mass production and produces a honeycomb structure having a reduced partition wall thickness by improving formability (particularly fluidity) during extrusion. This document discloses adding 2 to 6 wt % of a reversible thermally gelling binder to a cordierite-forming raw material batch, the reversible thermally gelling binder being prepared by mixing a reversible thermally gelling binder having a viscosity (2% aqueous solution) (20° C.) of 5000 cp or less and a reversible thermally gelling binder having a viscosity (2% aqueous solution) (20° C.) of 20,000 cp or more, the mixing being performed in a range of 10/90 wt % to 50/50 wt %.
Japanese Patent No. 3227039 aims at providing a method of producing a cordierite honeycomb structure that can maintain fluidity when the ceramic clay passes through the die while improving a shape holding capability, and can prevent occurrence of cracks during drying. This document discloses adding 1.5 to 4 wt % of hydroxypropoxyl methyl cellulose and 0.5 to 3 wt % of methyl cellulose to a cordierite raw material so that the total amount of the hydroxypropoxyl methyl cellulose and the methyl cellulose is 2 to 7 wt %, the methyl cellulose having a viscosity (2% aqueous solution) (20° C.) of 8000 cp or more, the hydroxypropoxyl methyl cellulose having a viscosity (2% aqueous solution) (20° C.) of 8000 cp or more and a value obtained by subtracting the number of moles of hydroxypropoxyl groups added per glucose ring unit from the average number of hydroxyl groups in the glucose ring unit that are substituted with a methoxy group being 1.5 or more.